arrow-cpp: search for Thrift using pkg-config instead of cmake
arrow-cpp: search for Thrift using pkg-config instead of cmake

Setting it to "AUTO" allows for the first[1] find_package to fail
through to pkg-config search logic.

[1] https://github.com/apache/arrow/blob/2ae84e8e90a7c0d8cd37df07d69e982dcfd16eaf/cpp/cmake_modules/ThirdpartyToolchain.cmake#L228

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs